Sonnet 5.5 feels like a cocky rookie. It's capable, but it often makes mistakes and especially poor assumptions, and when I point it out it just goes "yeah I did that wrong". Opus is much better at judging assumptions, makes less poor assumptions and mistakes. And when I point them out it takes it to heart and makes sure it guards against such a mistake in the future.

Sonnet is great for simple work, such as subagents running a program and summarizing the output. Opus is just that much better at the more challenging work I find.

You can probably get Sonnet to reach the same goal, but Opus needs much less hand-holding and overwatch.